NEW YORK (Reuters Health) - Homeopathic zinc nasal sprays don't fight colds, and they probably cause some people to lose their sense of smell, the authors of a new analysis conclude.

"Increased Food and Drug Administration oversight of homeopathic medications is needed to monitor the safety of these popular remedies," Drs. Terence M. Davidson and Wendy Smith of the University of California, San Diego, and the Veterans Affairs San Diego Health System write in the July issue of the Archives of Otolaryngology -- Head and Neck Surgery.

While over-the-counter zinc gluconate sprays remain a popular cold remedy, a number of well-designed studies have shown that the sprays don't work, the researchers point out. There's also growing evidence that zinc nasal sprays could dull people's sense of smell, or even eliminate it, possibly permanently, they add.

To investigate whether zinc sprays could actually be causing users to lose their sense of smell, the researchers looked at 25 people who lost their sense of smell soon after using zinc gluconate gel. All had sought care at the university's Nasal Dysfunction Clinic. They also reviewed the medical literature on the relationship between zinc use and olfactory problems.

The researchers used a set of nine requirements known as the Bradford Hill Criteria to determine whether zinc caused the patients' olfactory problems. Researchers developed the criteria in 1965 so they could test whether tobacco smoking causes lung cancer without having to perform a large - and unethical - study in which they assigned some people to smoke. The seven criteria require that several people in different parts of the world see the same relationship, that the effect occurs soon after the potential cause, and other findings.

Davidson and Smith found the zinc-olfaction relationship fulfilled all nine criteria. For example, people report a loss of sense of smell within minutes to hours after using the spray; also, the relationship between zinc use and loss of sense of smell has been reported by several independent groups of researchers.

"Based on our analysis, it appears evident that intranasal zinc can and does cause anosmia," the clinical term for loss of sense of smell, the researchers say.

Right now, they add, homeopathic remedies aren't required to follow FDA rules on expiration dating or laboratory testing to confirm that they actually contain the active ingredients they claim, at the stated strength.

"Protecting our patients from the potential risks of intranasal zinc medications and other homeopathic drugs, especially ones with limited therapeutic benefit, should be a high priority of the FDA," Davidson and Smith conclude.